Line mounted fault detectors (also known as faulted circuit indicators or FCI's) are used in distribution circuits to identify electric power lines where high current faults have occurred. Overcurrent detectors in distribution voltage circuits at electric utilities are commonly used to detect faults that produce significant increases in current. This works well for many faults because many distribution circuit faults cause currents well in excess of the normal load current. An FCI is commonly mounted directly to an individual phase conductor and is usually installed there by a technician using an insulating stick called a “hot stick” to install and remove the device from the power line while the conductor remains energized.
Although conventional FCI are effective at detecting low-impedance, high-current faults, they are not effective for detecting high-impedance, low-current faults even when they send their information to a common monitoring point. This is because high-impedance faults, for example where the fault current is less than about 1.5 times the normal current, remain below the triggering threshold of the overcurrent FCI. As a result, conventional FCI technologies only detect and report high-current fault events.
Some FCIs have the capability of storing data logs and providing “event” reports giving full current waveform data, with each FCI providing a log for its associated phase conductor (i.e., event reporting on a single-phase basis). In addition, there are conventional overcurrent devices with the ability to determine the location of faults on radial lines, but they only trigger for high-current faults. As a result, virtually all of the current technologies utilized on power lines for detecting and locating low-impedance, high-current faults are ineffective for detecting high-impedance, low-current faults.
For high-impedance fault detection, conventional approaches used in substations require the use of three synchronized current sensors and synchronized voltage signals to determine the existence of a high-impedance fault. But even with three-phase synchronized information available, conventional technology is not capable of determining the location of the fault or isolating the specific line segment where the fault has occurred on power lines with multiple tap points. Moreover, existing fault monitoring techniques located between the substations are not able to detect the presence of high-impedance faults, which limits high-impedance fault location detection to devices located in substations.
High-impedance faults can generally be defined as faults where the fault current is less than about 1.5 times normal phase current. Current FCI technology used for overcurrent protection is typically not capable of providing any indication of the presence these high-impedance faults. When high-impedance faults occur, which account for about 70% of faults, they present significant safety concerns to the public because they are often caused by energized lines touching trees or broken, still energized lines touching the ground creating significant electrical hazards. Since high-impedance faults are not detectable by conventional overcurrent FCI sensors, their location can be nearly impossible to find. At present, while detection at substations is possible, determination of the location down to a specific segment of distribution line is presently accomplished in most cases by visual inspection of arcing or the fires they cause.
Single-phase current monitors are not capable of determining the location of these dangerous high-impedance faults. While some currently available techniques have been used to detect the presence of high-impedance faults by detecting a harmonic signature characteristic of a fault caused by “arcing” in the faults, these techniques do not determine the location or direction to the fault. Finding the location of the fault requires some hint at which direction the fault is located otherwise the entire line must be inspected. In addition, visual inspection also has limits because things like cracks in insulators can be difficult to find visually, for example when a crack is on the other side of the insulator from the line of sight.
The result is that a distribution circuit can experience a high-impedance fault and the utility crew may not be able to even detect the presence of the fault because the overcurrent detectors typically installed on distribution lines do not respond to the current levels created by the high-impedance fault. Even when more sophisticated (and expensive) equipment is installed to detect the presence of high-impedance faults, the location or the direction to the fault from the monitoring equipment cannot be detected. With currently available technology, detecting the location or the direction to the fault from the monitoring equipment presently requires even more expensive solutions.
As a result, there is a persistent need for a lower cost solution to detecting high-impedance faults on distribution circuits to increase adoption of the technology and achieve the associated public safety benefits.